ABSTRACT Tourette syndrome (TS) is a neurodevelopmental disorder, affecting 0.5-1% of the pediatric population, with a marked male predominance. TS is characterized by multiple, recurring tics, which are a source of significant disability. The clinical management of TS poses considerable challenges, partially due to the limited efficacy and numerous side effects of available pharmacotherapies. Tics are triggered and exacerbated by stress, but the underpinnings of this association remain unclear. This proposal seeks to test a novel candidate mechanism whereby stress exacerbates tics, which may represent a new therapeutic target. Over the past few years, the Bortolato lab has shown that neurosteroids, a class of brain-produced steroids that play a central role in the orchestration of stress response, are involved in the neurobiological mechanisms of tic modulation. In particular, our preliminary data in animal models strongly suggest that the adverse effects of stress on tic severity are mediated by the neurosteroid allopregnanolone (AP). The implication of neurosteroids in TS is further suggested by our findings on finasteride, an inhibitor of the key steroidogenic enzymes 5?-reductase 1 (5?R1) and 2 (5?R2). In preliminary clinical trials, this drug led to a significant reduction of tic severity in adult TS patients refractory to standard treatments; furthermore, in animal models of TS, this drug reduced both stereotyped movements and other TS-related behavioral deficits. Although these data are promising, finasteride is contraindicated in childhood due to its demasculinizing effects. Nevertheless, preliminary data in animals suggest that the therapeutic effects of finasteride may be due to 5?R1 inhibition, while the antiandrogenic effects of this drug are primarily due to 5?R2 inactivation. Based on this background, our central hypothesis is that acute stress triggers tics by increasing 5?R1- mediated synthesis of AP. To test this hypothesis, we will use a mouse model of TS firmly grounded in tic pathophysiology, recently developed by the Pittenger group. This model, which reproduces the depletion of striatal cholinergic interneurons observed in post-mortem samples of subjects with severe TS, is particularly well-suited to test our hypothesis, since it exhibits tic-like behaviors in response to stress. The central hypothesis will be tested in two aims, to study how AP (Aim 1) and 5?R1 (Aim 2) contribute to the tic-like responses induced by stress in this model. The proposed research will be the first to test a specific mechanistic hypothesis explaining how acute stress exacerbates tics in TS. If our hypotheses are confirmed, these experiments may lead to a novel therapeutic approach for tics, since highly specific 5?R1 inhibitors have already been developed for clinical use and have excellent tolerability. Furthermore, because neurosteroids modulate the neuroactive functions of sex hormones, a better understanding of their role in tic genesis may help explain the male predominance of TS.